Tag Info

Hot answers tagged

18

There are two things that can be considered: one is trivial - that it is quieter at night so you are more likely to hear the horn. The second is physics: the speed of sound depends on the square root of temperature, so the refractive index is proportional to $T^{-1/2}$. At night it is quite possible to get a temperature inversion, such that air near the ...


7

Sound waves do generate changes in temperature because the propagation of sound is an approximately isentropic process. Keep in mind though that changes in static temperature can very well occur without the generation of heat. Moreover, the pressure changes associated with sound waves are of such a small magnitude that the observable temperature changes are ...


7

It's a rather late answer, but the paper Visible optical beats at the hertz level has just appeared on the Arxiv and this describes exactly the phenomenon you ask about. This image from the paper shows the experimental setup: The light frequency is modified using acousto-optic modulators (labelled AOM in the diagram), and to make it look pretty a lens is ...


6

Sound is attenuated in air - this is an irreversible, lossy process that results in the heating of the air. You can conclude that it is not an isentropic process. See my recent answer to another question for some of the math behind this - showing that while the amount of heating is very small, it is not zero. For practical purposes (for example, for ...


5

Put more simply: sound waves are attenuated as they propagate through air (this is more easily measured for very short wavelengths, e.g. ultrasound). This means they lose energy - which is turned into heat of the air. The amount of heating, however, is very very small. Let's do the math. A sound wave of 120 dB (really loud) has energy of only $1 ...


4

It might not have to do with day/night physics at all. It is possible that the train doesn't run during the day so it is unlikely you would even hear the horn. I live in a city, about 2.5 miles from the rails, where during the day the train tracks are used only for public light rail transit. The large locomotives only run at night after the light rail ...


4

In principle, sure. That's what microphones are, as ACuriousMind points out. But if you want to power anything substantial, an important issue to overcome is the relatively small amount of energy contained in sound waves. According to this website, the front rows of a rock concert have a sound intensity of $10^{-1}~\text{W m}^{-2}$. So even if you had a ...


4

The different tonality of a note in different instruments stems from the different mixes of amplitudes in the harmonic frequencies that the instrument provides. To be more concrete (and keeping to a slightly simplified view), you play the A note (440 Hz) and then you have the harmonic frequencies 880, 1320, 1760, ... ($440n$ where $n$ is the number of the ...


4

The explanation of alemi is correct. I noticed this in 2006 and had a first-year do a study of it: http://www.math.dartmouth.edu/~ahb/notes/Paopao2007poster.pdf (the spectrograms are not good in that poster). Here's a spectrogram we recorded from a hand-clap in the standard 20'x20'x40' court showing two clear slopes: We also found foot-stamping as a good ...


4

It seems that the harmonic (integer multiple) overtones of a sound usually all have the same phase. Is this true...? No, I don't think this is generally true, although it may be true for certain instruments. What led you to believe this? In trumpet tones, for example, the different harmonics come up at different times during the attack, so it seems ...


3

There are several explanations, depending on a lot of factors. At night, the air near the ground can have a different temperature than air only a few hundred feet above1. This affects the transmission of sound waves. There is often less wind at night. If the wind is blowing towards the train it can be harder to hear There is usually less ambient noise ...


3

If the bell is still vibrating when you let air inside it, then the answer is yes. If the bell was damped just before the door is opened, then the answer is no. Sound is transmitted through compression / decompression waves (pressure waves) in a medium (e.g. air, water, wall). This necessitates contact of the vibrating source of sound with such a medium. ...


3

Heat corresponds to random movements of atoms and molecules. It travels only through conduction - slowly. Sound consists of ordered movements, travelling through a medium as a wave (although it can also stand still, as in a standing wave). Large numbers of atoms or molecules move back and forth in synchrony. Sound eventually becomes random, as it is ...


3

A plank is a complicated example to choose because it's a composite material with a complicated structure. A better choice would be a piece of iron or some other homogeneous material. In that case the speed of sound is given by: $$ v = \sqrt{\frac{K + \tfrac{4}{3}G}{\rho}} $$ where $K$ is the bulk modulus and $G$ is the shear modulus. The bulk modulus is ...


3

It's not just a pure single frequency of sound that is being transmitted by an instrument. Just like with light, if you ask the frequency of the sun's emission, the answer would be that it's a whole broad spectrum (hence its ability to produce a rainbow, or allow objects to reflect colours other than yellow) but its peak frequency is yellow. You can ask for ...


3

For a long time, timbre was believed to be based on the relative amplitudes of the harmonics. This is a hypothesis originally put forward by Helmholtz in the 19th century based on experiments using extremely primitive lab equipment. E.g., he used Helmholtz resonators to "hear out" the harmonics of various sounds. In reality, the relative amplitudes of the ...


3

Isentropic processes are ones with constant entropy. Since entropy is defined as dS = dQ/T, then a reversible adiabatic process with dQ = 0 is an isentropic process. Need to take a step back to understand this. First, the physics of waves in gases come from the fluid equations. These include conservation of mass, momentum and energy. These three ...


2

I guess you mean to ask - is the amplitude of the vibration proportional to the speed of the sound waves it produces? The speed of sound in an ideal gas for relatively small amplitudes ($\frac{\Delta P}{P} \ll 1$) is $v=\sqrt{\frac{\gamma P}{\rho}}$ where $\gamma$ is the adiabatic constant (i.e. $PV^\gamma=const$), P is the average pressure, and $\rho$ is ...


1

Paper cones were originally chosen for their rigidity and lightness, so they can move air quickly without deforming and couple to a motor easily at the center while also being easy to suspend from the basket by their perimeter with a simple corrugation or foam/rubber surround. Physics only played a major part in the ease of construction and performance was ...


1

To expand on Xcheckr's answer: The full equation for a single-frequency traveling wave is $$f(x,t) = A \sin(2\pi ft - \frac{2\pi}{\lambda}x).$$ where $f$ is the frequency, $t$ is time, $\lambda$ is the wavelength, and $x$ is position. This is often written as $$f(x,t) = A \sin(\omega t - kx)$$ with $\omega = 2\pi f$ and $k = \frac{2\pi}{\lambda}$. If you ...


1

I can't tell what exactly you may have heard, but sounds reflect off of all kinds of things, particularly flat walls, so echos from a loud sound like thunder is quite plausible. 5 seconds is about 1 mile of total propagation, so that again is plausible. Lightning is a large current that ionizes the air in its immediate vicinity. Such ionized air is ...


1

There's not much difference. Thermal vibrations would be perceived as sound (noise) if they were intense enough, but they are not. The thermal vibration amplitudes at room temperature are small enough that the ear is not sensitive to them. I've been told that the sound pressure level for thermal vibrations is close to, but below, the threshold of ...


1

If a system returns to the initial conditions after a process, then that process has been reversed and (assuming one of the initial conditions is the temperature) is isentropic. Once a sound ends, the molecules remain in essentially the same position as before the sound. Drastic increases or decreases in the number of molecules at the source (explosions) ...


1

A qualitative picture of what happens in a gas can be made in terms of whether the behavior is random or non-random, oscillatory or steady. Temperature describes the random motions of the particles that comprise some object. Correlations, if they exist, disappear rapidly with distance between particles. In an ideal gas, correlations don't exist, period. ...


1

The statement in the first paragraph "In fact, today’s standard turbines are based on the same physical principles as 18th century windmills." is marketing hooey. They are hanging their hat on the fact that the windmills were unducted props and most of today's turbines are the same, which is true. The airfoils used today are not 18th century designs. They ...


1

There's an error in that the type of pipe for each of the two fundamental frequencies as described in your comment don't match the problem description. The pipe with a fundamental frequency of 440Hz is open-closed, and the pipe with a fundamental frequency of 660Hz is open-open. You actually said "closed-closed", which isn't even an option, but even if ...



Only top voted, non community-wiki answers of a minimum length are eligible